The public switched telephone networks (PSTN) are analog networks designed primarily for the transmission of voice information. Through the use of modems these networks are capable of transmitting data, although even using the state of the art technology the data bandwidth is very limited. In order to meet the demand for increased bandwidth, a digital network known as integrated services digital network (ISDN) has been developed.
The PSTN is a system whereby calls (and thus circuits used to make the connection) are dedicated until either party terminates the call. This system is very inefficient because any unused bandwidth of a given call cannot be shared with other parties. Thus the PSTN may have thousands of circuits that are being underutilized and still not be able to have enough circuits to satisfy all the callers at a given time.
The ISDN is a switched digital network that enables the local telephone company to provide a range of services in order to meet the needs of their users and enable the telephone company to utilize their resources (circuits and switches) efficiently. A switched network is a network that provides a choice of services for establishing a circuit or network connection between parties. Those services typically include circuit switching services and packet switching services; however, other services such as asynchronous transfer mode (ATM) or frame relay technology are also available in some areas. The ISDN provides the available services to the user on an as needed basis as well as enables the user to increase or decrease bandwidth on demand.
The ISDN is intended to be a worldwide public telecommunications network to replace the existing PSTN and provide a wider range of services than the PSTN. In order to facilitate international standardization of ISDN systems, the Telecommunications Standardization Sector of the International Telecommunications Union (ITU-T) has promulgated the I Series Recommendations. The ITU-T I Series Recommendations in combination with other more general networking ITU-T recommendations define many aspects of the ISDN services, including the network interfaces and protocols.
ISDN service usually comprises one D-channel and one or more B-channels. The D-channel is used to setup the call with the ISDN service provider central office and to provide all the call control signals for each of the B-channels. The user contacts the telephone company via the D-channel to request different types of services provided by the network. Typically, the D-channel data rate can be 16K or 64K bits per second (bps). The Bearer Channel or B-channel is the basic user channel for the transfer of data and can provide a data rate of up to 64K bits per second (bps) per channel. It can be used to carry digital data, digitized voice, or a combination of the two. More than one B-channel can be combined to achieve higher data rates. Essentially, three kinds of circuit or network connections can be setup over a B-channel: circuit switched, packet switched and semipermanent (equivalent to a leased line). In addition to or instead of B-channels, H-channels, providing data rates greater than 64 Kbps, can be provided.
Basis ISDN access service, intended for residential and small office subscribers, is provided by what is called a Basic Rate Interface (BRI) which typically consists of two B-channels and one D-channel. A Primary Rate Interface (PRI), intended for commercial subscribers, typically consists of 23 or 30 B-channels and one D-channel. The PRI may also include H-channels instead of or in addition to B-channels.
The ITU-T recommendations define network interfaces and protocols in terms of the Open Systems Interconnect (OSI) reference model. The OSI reference model is an internationally recognized standard developed to provide a common basis for the coordination of standards development for systems interconnection. The OSI reference model separates the communications functions of a system into a hierarchical set of seven layers. Each layer performs a related subset of the functions required to communicate with another system. Each layer relies on the next lower layer to perform more primitive functions (and conceal the details of those functions) as well as provides services to the next higher layer. The seven layers are the physical layer, the data link layer, the network layer, the transport layer, the session layer, the presentation layer and the application layer. The physical layer, the data link layer and the network layer together provide what may be called network services. The ISDN interfaces and protocols are primarily concerned with these three layers.
The physical layer is concerned with the mechanical and electrical connections that couple the transmission medium (usually copper cable or optical fiber) that make up the circuit. The physical layer is also concerned with how an unstructured bit stream is transmitted across over the physical link. ITU-T Recommendations I.430 and I.431 define the physical layer for the ISDN Basic Rate Interface (BRI) and Primary Rate Interface (PRI) respectively. Examples of other physical layer specifications include Electronic Industries Association (EIA) RS-232 C, RS-422 and portions of ITU-T X.21
The data link layer provides for reliable transfer of data across the physical link and for the transmission of blocks of data (frames) with the necessary synchronization, error control and flow control. One common data link layer protocol is High-level Data-Link Control (HDLC). Other related data link layer protocols include Link Access Procedure Balanced (LAPB) for packet switched networks, X.75/SLP (Single Link Procedure) is similar to LAPB but provides additional features for interconnection of switched networks, Link Access Procedure for the D-channel (LAPD) used on the D-channel for data link control, ITU-T V.120 (similar to LAPD) used on the B-channel for circuit switched connections, Link Access Procedure for Frame-mode bearer services (LAPF) for networks providing frame relay services and Logical Link Control (LLC) for local area networks.
The network layer provides for the transfer of information between end systems across a network and relieves the upper layers of the burden of having to accommodate the underlying network protocols and technologies. The network layer is responsible for establishing, maintaining and terminating network connections. One example of a network layer standard is ITU-T X.25.
Circuit switched networks provide a dedicated circuit (on demand) between parties when either party places a call. The call is established when the network creates an end-to-end circuit connection between the parties. Once the call is established, the parties are free to select any data link protocol for the transmission of data. ITU-T Recommendation I.465/V. 120 (hereinafter referred to as V.120) defines one data link layer control protocol for circuit switched network services.
Packet switched networks provide a network of pathways between the parties and the system breaks the data transmitted between the parties into packets that are routed through the network. One of the most widely used protocols for packet switched network services is defined in ITU-T Recommendation X.25. ITU-T Recommendation X.25 defines one well known data link layer control protocol, Link Access Procedure--Balanced (LAPB).
In addition to V.120 and LAPB, other data link layer control protocols that can be used on switched networks include HDLC, transparent HDLC Synchronous Point-to-Point Protocol (Synchronous PPP), transparent HDLC Asynchronous Point-to-point Protocol (Asynchronous PPP), LAPF and LLC.
In order for two parties to transfer data, it is necessary for the parties to establish both a circuit or network connection between them and to establish a data link connection between them. As used herein, the circuit or network connection is intended to refer to the physical layer link between the parties equipment that affords the transfer of data. The data link connection includes the procedures and protocols necessary for the parties to transfer data via the circuit or network connection. A data link connection may be initiated by either party, typically by initiating a link setup procedure whereby the parties agree to begin the exchange of data according to a common protocol.
In order for a data link connection to be established it is necessary for both parties to utilize the same data link layer protocol. Thus, the parties must agree on the data link layer protocol prior to placing the call. This creates a problem for users of the types of networks that provide many services because there are many possible data link protocols. If the parties do not specify a data link protocol prior to placing the call and the parties' network interfacing equipment is configured for different data link protocols, a data link connection may not be established.
One solution to this problem is to assign separate telephone numbers for each protocol supported. Another solution is to utilize a sub-address to direct the call to a port configured for a specific protocol. The disadvantage with these solutions is that the caller still has to contact the remote party to determine which telephone number or sub-address is assigned to which protocol. In addition, some networks do not support sub-address functions.